1. Field of the Invention
This invention relates to generation of millimeter radiation. In particular the invention pertains to an integrated millimeter radiation source and waveguide.
Many uses of microwave radiation, such as collision-avoidance radars, have large volume markets that require low cost components. One approach to satisfy these requirements is based on integrated circuit antennas. These antennas radiate primarily into the substrate and a problem often encountered is loss of energy to surface waves, resulting in a loss of antenna efficiency.
This invention involves a unique geometry of the substrate such that as much of the energy as possible is coupled to surface waves--i.e., the energy is waveguided and then emitted at an aperture. This approach is particularly appropriate for a distributed antenna based on transmission lines (e.g., coplanar transmission lines).
2. Description of the Prior Art
Reference is made to Infrared and Millimeter waves, vol. 10 chap. 1, "Integrated--Circuit Antennas", Rutledge et al., Academic Press, 1983. This reference describes various configurations used for making integrated circuit antenna. It also discusses some of the limitations to antenna efficiency, including energy lost as surface waves.
Grischkowsky et al., "Electromagnetic Shock Waves from Transmission Lines", Phys. Rev. Lett. 59, 1663 (1987) describes a particular coplanar transmission configuration where pulses on Aluminum lines deposited on a Silicon Sapphire (SOS) substrate radiate into the substrate at an efficiency dependent on the characteristic frequency.
Reference is made to M. B. Ketchen, et al., "Generation of Subpicosecond Electrical Pulses on Coplanar Transmission Lines", Appl. Phy. Lett. 48 (12), Mar. 24, 1986, pp.751-753. This publication describes techniques to generate ultrashort electrical pulses by photoconductively shorting charged transmission lines across narrow gaps.
The generation of fast electrical pulses utilizing photoconductive resonant cavities has also been proposed in the literature. Reference is made to IBM Technical Disclosure Bulletin Volume 31, No. 12 pp. 392-393, May 1989. This publication discloses the use of a photoconductive resonant cavity in which the cavity length is matched to the repetition rate of the excitation optical source, such as a semiconductor diode. The resonant cavity is defined by having impedance discontinuities in the transmission line and is selected so that the round trip time of an electrical pulse is equal to the period of laser oscillations or a multiple thereof. The discontinuity presents 100% mirror at one end of the cavity. Thus, the internal pulse amplitude within the cavity is significantly larger than in a non-resonant cavity. The resonant cavity formed in this publication is an electrical analog of a Fabry-Perot.
Thus, while the art provides alternative concepts for the generation and guiding of millimeter waves, a need still exists in the art to define more efficient techniques of guiding radiation from a distributed microwave emitter, such as a transmission line, such that all of the radiation is emitted as a point source. That is, the waveguide should be used whose ends can act as a point source.